1. Field of the Invention
The present invention relates to an optical pickup device, in particular, an optical pickup device used in an optical disc device for optically recording and reproducing information.
Recently, the optical disc device has been made thinner, i.e., have reduced height. In response to this tendency, it is required to made the optical pickup device thinner or of less height.
2. Description of Related Art
FIG. 1 shows an optical pickup device 10 of the related art. The optical pickup device has an arrangement effective for thinning the device. For this purpose, a light emitting unit 11 is provided in which a semiconductor laser, a light detector, a diffraction grating, and a hologram element for performing deflection and light dividing functions are integrated (not shown in the figure). Further, a limited-distance objective lens 12 is provided so as to eliminate use of a collimator lens. A height h.sub.0 of the optical pickup device 10 is the distance between a bottom or lower end of the light emitting unit 11 and a label surface (the top surface) of an optical disc 15 as show in the figure. Further, in the figure, directions D.sub.1 and D.sub.2 indicate the directions of movement of the objective lens 12 during a focusing control operation and thus an image point 18, relative to an object point 17, is positioned on the label surface of the disc 15. A level Hc indicates the neutral position in the objective lens 12.
Three light beams 13 emitted from the light emitting unit 11 are reflected by a stand-up mirror 14 and, thus, are vertically deflected. The stand-up mirror has a stand-up angle .o slashed..sub.0 set to be 45 degrees. The deflected light beams 13 are then converged by the objective lens 12 and thus three minute spots (not shown in the figure) are projected onto a recording surface 15a of the disc 15. The light beams are then reflected by the recording surface 15a of the disc 15. Resulting reflected beams 18 are returned to the light emitting unit 11 via the objective lens 12 and the stand-up mirror 14.
The returned light beams are guided to the light detector (not shown in the figure) and then a focus error signal, a tracking error signal and a reproduced signal are obtained. The focus error signal is used in driving an actuator (not shown in the figure) and thus the objective lens 12 is moved vertically (along the D.sub.1 /D.sub.2 directions) in the focusing control operation. Thus, a focus error of the light beams on the recording surface 15a of the disc 15 is eliminated. The tracking error signal is used in driving another actuator (not shown in the figure) and thus the objective lens is moved horizontally. Thus, tracking error of the light beams on the recording surface 15a of the disc 15 is eliminated.
FIG. 2 shows another optical pickup device 20 in the related art. Reference numerals which are the same as those of components shown in FIG. 1 are used for corresponding components shown in FIG. 2. The optical pickup device 20 is disclosed in Japanese Laid-Open Patent Application No. 3-189932 and Japanese Laid-Open Patent Application No. 5-120723.
In the optical pickup device 20, the stand-up mirror 14 has a stand-up angle .o slashed..sub.1a set to a value smaller than the above-mentioned stand-up angle .o slashed..sub.0 of 45 degrees shown in FIG. 1, for example, 40 degrees. As the stand-up angle .o slashed..sub.1a becomes smaller value, the light emitting unit 11 can be raised as show in FIG. 2 and also a height .sub.a.sub.0a of the standing mirror 14 decreased from the height a.sub.0 shown in FIG. 1. As a result, the bottom end of the light emitting unit 11 is higher than the bottom end of the standing mirror 14 as shown in the figure. Therefore, the height h.sub.0a of the optical pickup device 20 is lower than the height h.sub.0 of the optical pickup device 10 shown in FIG. 1. In other words, the optical pickup device 20 is thinner, i.e., is of less height, than the optical pickup device 10.
A relationship of the stand-up angle .o slashed..sub.1a of the stand-up mirror 14 and an angle .o slashed..sub.2a by which the light beams are deflected is indicated by an equation .o slashed..sub.2a =2.o slashed..sub.1a. Accordingly, in order for the light beams reflected by the stand-up mirror 14 to have the stand-up angle smaller than 45 degrees and be precisely vertically deflected, the light emitting unit 11 is inclined as shown in FIG. 2. Thus, a light axis 11a of the light emitting unit 11 is downward inclined at an angle .o slashed..sub.3a from the horizontal direction.
An objective-lens holder 21 encircles the objective lens 12 and thus provides support. The objective-lens holder 21 has an aperture stop 21a at the bottom end thereof for limiting light beams which are reflected by the stand-up mirror 14 and then incident on the objective lens 12. The limited-distance objective lens 12 has a convex top surface 12a and a convex bottom surface 12b. The convex top surface 12a has a top portion 12c thereon at a position at which the light axis of the objective lens 12 crosses the convex top surface 12a. The top portion 12c is located at a level H.sub.1 as show in the figure. The convex bottom surface 12b has a bottom portion 12e thereon at a position at which the light axis of the objective lens 12 crosses the convex bottom surface 12b. The bottom portion 12e is located at a level H.sub.2 as show in the figure.
In the optical pickup device 20, the aperture stop 21a of the objective-lens holder 21 is located lower level than the level H.sub.2 of the bottom portion 12e of the objective lens 12. The light emitting unit 11 is positioned at a level H.sub.3, and the top end 11a of the light emitting unit 11 is located lower than the level H.sub.1 of the top portion 12c.
Considering that a very thin optical disc device will be incorporated in a notebook-type personal computer, further thinning or height reduction of the optical pickup device 20 shown in FIG. 2 is required. For this purpose, it can be considered that the position of the stand-up mirror 14 is further raised and/or the stand-up angle .o slashed..sub.1a is further reduced. In order to make clear problems which may occur as a result of such modifications being performed, for example, FIG. 3 shows an arrangement resulting from performing the modifications on the device shown in FIG. 2. In FIG. 3, `Mr` indicates a movement range of the holder 21.
As shown in FIG. 3, the height of the optical pickup device is reduced to a height h.sub.0aa as show in the figure, and thus the device is of reduced height. However, when the objective lens 12 is moved downward in the focusing control operation, a part of the light beams 13 is blocked by the bottom end of the objective-lens holder 21. Thus, a partial eclipse occurs and thus information reproduction from the optical disc 15 may not be properly performed.